High tensile steel for large heat-input automatic welding and production process therefor

ABSTRACT

A high tensile strength steel for a large heat-input welding more than 50,000J/cm which contains more than 0.004% by weight in respect to steel weight of titanium in TiN phase smaller than 0.05 Mu in the steel before welding and comprises 0.03-0.23% of carbon, 0.02 to 0.8% of sulphur, 0.5 to 2.0% of manganese, 0.004 to 0.07% of titanium 0.0005 to 0.10% of soluble aluminium, and 0.003 to 0.012% of total nitrogen, with addition of at least one selected from the group consisting of B, V, Ni, Cu, Cr and Mo.

United States Patent [1 1 Kanazawa et al.

[ Nov. 20, 1973 [73] Assignee: Nippon Steel Corporation, Tokyo,

1 Japan 22 Filed: Mar. 24, 1971 21 Appl. No.: 127,710

[30] Foreign Application Priority Data Mar. 26, 1970 Japan 45/25042 Oct. 15, 1970 Japan 45/90636 [52] US. Cl. 75/123 B, 75/123 J, 75/123 K,

75/123 M, 75/124, 148/2, 148/3, 148/36 [51] Int. Cl..... C22c 39/02, C22c 39/54, C21d 7/14 3,368,887 2/1968 Enis et a1. 75/128 3,615,904 10/1971 Kindlimann et a1. 148/12.l 3,432,368 3/1969 Nakamura 148/12 3,625,780 12/1971 Bosch et al. 148/12 X 3,163,565 12/1964 Wada 148/143 12/1970 Zanetti 148/12 OTHER PUBLICATIONS F. P. H. Chen, Dispersion Strengthening of Iron Alloys by Internal Nitriding, University Microfilms, lnc., Ann Arbor, Michigan (via Rensselaer Polytechnic Institute, Troy, N.Y., 1965), pp. 75-87.

Primary Examiner-W. W. Stallard Att0rney-Toren & McGeady [5 7] ABSTRACT A high tensile strength steel for a large heat-input welding more than 50,000J/cm which contains more than 0.004% by weight in respect to steel weight of titanium in TiN phase smaller than 0.05 p. in the steel before welding and comprises 0.030.23% of carbon,

[58] 0.02 to 0.8% of sulphur, 0.5 to 2.0% of manganese, 2 3 0.004 to 0.07% of titanium 0.0005 to 0.10% of soluble aluminium, and 0.003 to 0.012% of total nitrogen, with addition of at least one selected from the group [56] References Cited consisting of B, V, Ni, Cu, Cr and Mo.

UNITED STATES PATENTS 2,474,766 6/1949 Wa goner et al 75/ 123 H 11 Claims, 7 Drawing Figures Inventive Steel E 1.1.] Q '3 O- 8 C? A Q --4Q' I Q: E Q Q 2. E, ow o C! U 5 5: *a a Q L- (U 0) 2 I u.| '0 E. o s 2 I n U o y 6 9 E Q. N 0 Conventional Steel 0: 1 4 DC. .0 a

' o o o o Welding Heotinput( 10 /tm) Patented Nv. 20, 1973 I 3,773,500

7 Sheets-Sheet l FIG.1

i N VENTORS:

5/1060 HANAZAWA SHOJ' I SAITO ARIRA NAKHSHIHA NAZI/NARI VAATO KENTARO OKAHOTO HEN HANAYA' I ROUZI TANABE Patented Nov. 20, 1973 3,773,500

'7 Sheets-Sheet 2 amwuadwe Patented Nov. 20, 1973 3,773,500

'7 Sheets-Sheet 5 Conventional Steel 8 FIG. 3B

Inventive Steel A Cooling Rate at ingot Core (Pouring ll00C): 27.0C/min. Ileheating at 125oc Only one time Steel A Cooling Rate at. Inger. Core (louring 11ooc) 1 .5'C/min. Reheating At 1250'0 Only one time FIG. 3C

Structures of weld bonds by heat-input of 100,000 J/cm INVENTOR. 30060 KRNRIRWR SHGJI N70 GRIN! NRKRJINNR KRIUHRRI Munro "graze oxnnoro Patehted Nov. 20, 1973 3,173,500

7' Sheets-Sheet '4 FIG. 4A

in-national Stool I FIG. 4B

Ind-Q1" Sue] A cooling an. (Pouring -+l 100C) :ILO'G/lla :znuuq n uso'c v Only on. the F Shel A cool In an. ("nu -nave) LPG/nth Banana 0 1m 7 only on no FIG. 4C

Ht-Mucocpouad 1- a: mural INVENTOR. 050 knunznvn Juno sanefl l l ummsumn' Rnuumu ynnnro klurnno oxnnom KIN RRNIIR noun 'rawnu Merrie Absorbed Energy at O'C of Patented Nov. 20,1973 3,773,500

'7 Sheets-Sheet b FIGS Welding Heat-in ufl 10 %m) Patented Nov. 20, 1973 3,773,500

7 Sheets-Sheet 0 A E E2 ,6 8 C) D O 3 o a 8 w .r: c: 5 9 8 Averagevuiue u o ,c O U I N 3 "6 or 6 5 m c F 5. if, 2 w '52 .o u 5 l w '6 13 m :1:

steel sheet with no addition of Ti Contenfl'lby weight) of Tiin TiN under 0.05 44. in the material before welding Patented Nov. 20, 1973 3,773,500

7 Shoe Ls-Sheot 7 FIG.7

Q- Range in which toughness of weld bond by a large heal-input welding is particularly good Ti I.

HIGH TENSILE STEEL FOR LARGE HEAT-INPUT AUTOMATIC WELDING AND PRODUCTION PROCESS THEREFOR Thepresent invention relates to a high tensile steel for large heat-input'automatic welding and a production process therefor.

Conventionally, an automatic welding by a large heat-input has been widely used in practice in order to reduce the number of welding steps and welding cost. However, when the heat-input for welding is more than 50,000J/cm, toughness of weld heat affected portion, particularly rough grain bonds remarkably deteriorates and'causes a severe problem.

Therefore, in the conventional art, trials were made to prevent the lowering of toughness by restricting the heat-input for welding. Based on conventional knowledges, toughness of the welded bond is relatively good when the structure of the bond is a low-carbon martensite or a lower bainite, and when the welding heat-input increases and the amount of martensite decreases embrittlement increases gradually. Therefore it is necessary to limit the amount of welding heat-input below a certain value. And in order to prevent embrittlement in the weld bond even when a large heat-input welding is effected, it has been conventionally proposed to select the steel composition so that the bond has a high hardenability and its structure becomes martenstic as much as possible under the heat-input condition.

However, such conventional measures have many defects'. For example, the necessity of limiting the heatinput will limit a large heat-input welding itself, and the selection of steel composition for assuring a martenstic structure in the weld bond will require a large amount of alloying elements and naturally lead to a large increase of the equivalent carbon for welding (Ceq C +1724 Si+ l/6 Mn+ 1/5 Cr+ 1/4 Mo+ 1/14 v+1/40 Ni thus causing increased susceptibility to welding cracks and remarkable deterioration of welding ductility.

One of the objects of the present invention is to provide a high tensile strength steel which does not deteriorate its toughness in the weld heat affected zone particularly in the weld bond even when an automatic welding using a large heat-input more than 50,000J/cm.

Namely, the steel material obtained by the present invention is less susceptive to welding cracks and when a large heat-input welding of single layer or plural layers of more than 50,000J/cm is conducted, exhibits excellent toughness in weld bonds as compared with the conventional steels. 7 k

Further, the lower limit of a heat-input range for obtaining a particularly good toughness in the weld bond is 50,000J/cm while the upper limit of this range extends to even an electro-slag welding, and in this upper range the advantages and effects of the present invention are realized. This is one of the suprising features of the present invention.

Another object of the present invention is to provide a steel material having a low equivalent carbon content due to a smaller addition of alloying elements and excellent manual welding properties such as ductility in welded portions and weld cracking characteristics .in

spite of its high toughness in weld bonds at the time of a large heat-input welding.

Still another object of the present invention is to provide a steel material having a strength from SOkg/cm to kg/cm as required by a high tensile strength steel and an excellent low-temperature toughness of vTrs -20 to 1 10 C by a heat treatment such as annealing, normalizing and quenching-tempering.

Other objects of the present invention will be understood from the following examples and the attached drawings.

The present invention shall be described in details referring to the attached drawings: I

FIG. 1 is a schematic view showing the method for preparing a 2mm V charpy test piece from a welded portion.

FIG. 2 is a graph examples of weld heat cycle curves at single layer welding at 60,0001/cm and 100,000J/cm heat-inputs.

FIG. 3 is a photograph showing the structure of a bond produced by a sub-merged welding at 100,000J/cm.

FIG. 4 is an electron microscope photograph (by the extraction Replica method) of titanium compounds in the present inventive steel, a conventional steel and a comparative steel.

FIG. 5 is a graph showing changes in energy absorbed at 30 C by 2mm V charpy test piece of a reproduced heat cycle bond.

FIG. 6 is a graph showing changes in the absorbed energy of a reproduced heat cycle bond due to the titanium content in the TiN phase smaller than 0.05p..

FIG. 7 is a graph showing an appropriate proportion between the titanium content and the nitrogen content according to the present invention.

According to the present invention, it is necessary that a steel ingot or cast billet is cooled below l,l00 C at an average cooling rate of more than 5 C/min. from the temperature of molten steel at pouring down to l,l00 C so that the titanium compounds in the steel finer that those in a steel ingot which is cooled below l,l00 C at a cooling rate of less than 5 C/min. from the temperature of molten steel at pouring down to 1,100 C.

The above definition is one of the very important requisits of the present invention. The reasons for the above definition will be explained below.

Solidification or the cooling rate after solidification of a steel ingot or cast billet greatly affects the results of the present invention, and when the whole average from the temperature at the pouring of molten steel for ingot making to the solidification of the core portion of the steel ingot or cast billet and thereafter down to 1,100 C is more than 5 C/min., the toughness in weld bonds by a large heat-input welding shows particularly excellent value.

Thus, the cooling rate of the steel ingot or cast billet down to 1,000 C is very important, and this has a close relation with the steel composition. Namely, the present inventors have discovered that the cooling rate is very important to avoid gathering and excessive growth of titanium compounds which are formed in the steel and stable at high temperatures and to disperse the compounds as fine as possible so as to improve the structure of weld bonds produced by a large heat-input welding.

Namely, according to the studies conducted by the present inventors it has been found that the numerical proportion of fine titanium compounds less than 10,000 A to the total number of titanium compounds is more than 50% as observed by the extraction Replica method using an electron microscope (magnification X 10,000), spaces among the dispersed titanium compounds are less than 2 5n, and the toughness of weld bonds produced by applying a large heat-input welding to a steel under such conditions is remarkably good.

In other words, when the titanium in TiN less than 0.05p. of the total TiN present in the steel before welding is more than 0.004%, the toughness of weld bonds produced by a large heat-input welding is remarkably increased (see FIG. 6) (For the method of analysis of titanaium compounds in steels by sizes, see Tetsu to Hagane" Vol. 55, No.11 5693 and the postscript).

The above precipitation condition of the titanium compounds is attained when the average cooling rate down to l,l C after the pouring of molten steel is more than C/min., and on the contrary, when the average cooling rate is less than 5 C/min., the numerical proportion of the fine ltitanium compound less than 1,000 A becomes less than 50% under the electron microscope, and the percent of tianium in TiN smaller than'0.05p. is less than 0.004%, and the effect of improving the toughness of weld bonds by a large heatinput welding is also small.

The cooling condition for assuring the average cooling rate of 5 C/min. down to more than l,l00 C can not be obtained in ordinary steel ingots having more than 500kg weight of round, square, flat, polygonal or chrysanthemum shape, for example. Namely, in these cases the average cooling rate down to l,l00 C is less than 5 C/min.

Conditions of cooling the steel ingot below l,l00 C do not affect the results of the present invention.

Regarding the method for analysing the titanium compounds by sizes, the following unpublished method must be used.

Sam ale 1g Residue (titanium nitride smaller than 0.05

Filtrate (titanium in solid solution and titanium nitride smaller than 0.05u)

The term steel ingot" used in the present invention means steel lamp as cast with ordinary a casting mold, and the term cast billet" means steel lamp as cast by a continuous casting.

In general, the steel ingot or cast billet after cooling must be reheated for hot working. In case of heating the present inventive steel, it is note-worthy that if the frequency of the heating above l,050 C is not more than one time, the toughness of weld bonds by a large heat-input welding show particularly excellent value. This is due to the fact that titanium compounds which precipitate and grow excessively during the heating of 'the steel ingot or cast billet can be held in a finely divided form in the steel sheet before welding by selecting the heating condition as above.

Yet, what must be particularly noted in this point is that the limitation of the heating condition in the production step improve always the toughness of the weld bonds by a large heat-input welding irrespectively of the cooling rate of the steel ingot in the previous step.

Namely even if the cooling rate down to l,l00 C from the temperature of molten steel at the time of pouring below or above 5 C/min., the toughness of the weld bond by a large heat-input welding is improved so far as the frequency of the heating above l,050 C is not more than one time in the subsequent steps. For the reasons as set forth above, the heating conditions are limited in the present invention. However, the most preferable heating conditions vary slightly within the above limited range depending on the cooling rate of the steel ingot. Namely in the case of steel ingot or cast billet which is cooled below 1,100 C at an average cooling rate of more than 5 C/min. at the core from the temperature of molten steel at the pouring to l,l00 C, satisfactorily good results can be obtained provided that the frequency of heating above l,l00 C is not more than one time in the subsequent steps.

On the other hand, in case of a steel ingot which is cooled at an average cooling rate of less than 5 C/min. at the core from the temperature of molten steel at the pouring to l,l00 C, remarkably good results can be obtained when no heating above l,250 C is effected and the frequency of heating in the range of l,050 1,250C is limited not more than one time in the subsequent steps.

One point to be noted here is the selection of condition concerning under a cooling condition that the average cooling rate at the core of the steel ingot from the temperature of molten steel at the pouring to 1, C is less than 5 C/min.. This definition only explains the cooling condition and does not always mean to cool the steel ingot to l,l00 C. Namely, as usually done, when the hot ingot is charged in a soaking pit directly after ordinary ingot making, the temperature at the core portion of the steel ingot is not always cooled to l,l00 C.

Even in this case, the present invention includes a cooling condition under which the average cooling rate is considered to be less than 5 C/min. supposing the steel ingot is cooled to l,l00 C under the same condition as in the case when the steel ingot is directly cooled without being charged in a soaking pit. Namely, as heating temperature of steel ingot is limited to l,050 1,250 C in the present invention the temperature at the core portion of the steel ingot in case of hotingot charging does not lower to l,l00 C before the ingot is charged in a soaking pit at l,200 C for example. In this case, so far as the cooling rate of the steel ingot is considered to be less than 5 C/min. in the range from the pouring temperature is 1,100 C supporting the steel ingot is not charged in a soaking pit, this cooling condition is within the scope of the present invention.

Next, the reason for the definition of using as a starting material the steel which was produced without heating above l,245 C and with heating within the temperature range 1050 l,250 C not more than one time" will be explained below. As mentiond above, in case of ordinary ingot making, the heating temperature and the heating frequency thereafter willproduce large effects on the results of the present invention, and only single heating above l,250 C or more than one time of heating within the range of l,050 l,250 C will prohibit the achievement of excellent toughness in weld bonds of large heat-input welding as obtained in the present invention.

The frequency of heating, if the heating temperature is below 1,050 C, is not limited. Namely in the present invention, the heating condition above 1,050 C is very critical, and this has a very close relation with the steel composition defind by the present invention.

What must be noted in this point is that the above limitations of the cooling condition of steel ingots or the heating condition thereafter produce the remarkable effects on the toughness of weld bonds produced by a large heat-input welding only in case of the steel compositions according to the present invention. When a steel composition is outside the composition range defined by the present invention, no such effect is produced.

According to the results of the basic studies on the I present invention, the proportion by weight or the numerical proportion of the fine titanium compounds less than 1,000A particularly below 0.0511. in the steel and the space among the titanium compounds produce large effects on the structure and toughness in the weld bond and other welding heat affected zones when a large heat-input welding more than 50,000J/cm is conducted.

Next the reasons for composition limitations in the present invention will be explained.

The basic composition according to the present invention comprises C: 0.03 0.23%, Si: 0.02 0.8%, Mn: 0.5 2.0%, Ti: 0.004 0.07%, Al.sol: 0.0005 0.10%, total N: 0.0035 0.012%, with the balance being iron and impurities, and is suitable for a large heat-input welding. The reason for the above composition limitation will be explained below.

Less than 0.03% of carbon content, softening of the heat affected zone by a large heat-input welding is large and enough strength of this zone can not be obtained, and thus the lower limit of the carbon content is set as On the other hand, more than 0.23% of carbon content, the characteristics of the present invention that toughness of the weld bonds produced by a large heatinput welding is remarkably good is lost, and in addition embrittlement of. the weld bond is caused. Thus upper limit of the carbon content is set as 0.23%. The best toughness of the weld bond is obtained when the carbon content is 0.06 0.15%.

Also, when the carbon content is less than 0.03%, the strength of the material steel low, and when the carbon content is more than 0.23%, the so-called manual weldability (maximum hardness, susceptivity to welding crack and weld ductility) tends to deteriorate.

Silicon is an alloying element introduced inevitably by deoxidation, but less than 0.02% of silicon will not give enough toughness of the material and thus more than 0.02% of silicon is required. On the other hand, more than 0.8% of silicon deteriorates the toughness. From the reasons set above, the silicon content is limited to 0.02 0.8%.

Regarding manganese content, less than 0.5% of manganese content causes considerable softening of the heat affected portion and large shortness in the strength of this portion, and thus the lower limit is set as 0.5%.

When the manganese content exceeds 2.0% toughness of the weld bonds by a large heat-input welding ther improved.

more than 50,000J/cm sharply deteriorates and such material can not beuused in the present invention. Thus the upper limit of the manganese content is set as 2.0%. The best toughness of weld bonds is obtained when the manganese content is 1.1 1.8%. In case of the large heat-input welding more than 50,000J/cm, the toughness of the weld bond is particularly excellent if 10 X C%) (Mn%) 2 2.8. Further, the carbon content and the manganese content satisfy the condition of C 1/6 Mn 0.38, the toughness of the weld bond is still fur Next, regarding the titanium content, the characte ristics of the present invention that the toughness of weld bond is remarkably good when a large heat-input welding more than 5 X 10J/cm is applied can not be realized, thus the lower limit of the titanium content is limited as 0.004%. On the other hand, however, more than 0.07% of titanium, the toughness of weld bonds by a large heat-input welding deteriorates, and the toughness of the material itself deteriorates. Thus the upper limit of the titanium contet is set as 0.07%. The most desirable range of titanium content for the toughness of the weld bond is 0.015 0.04%.

Regarding soluble aluminum content, more or less than 0.0005% of it, the toughness of weld bonds by a large heat-input welding remarkably deteriorates, and thus the range of its content is limited to 0.0005 0.10%. The most desirable range of soluble aluminum is 0.0005 0.015%.

The reason for limiting the nitrogen content to 0.003 0.012% is that more than 0.012% of nitrogen remarkably deteriorates the toughness of weld bonds without exception. The best toughness of the heat affected zone is obtained when the total nitrogen content is 0.003 0.011%. This means that the nitrogen content is an indispensable element in the present invention. When the total nitrogen content is less than 0.003%, the toughness of weld bonds by a large heat-input welding lowers. Even when the total nitrogen content is less than 0.012%, particularly good toughness is obtained when acid insoluble nitrogen content is more than 0.005%. It is found from the results of experiments that the toughness of weld bonds is particularly good when the ratio of Ti to N is Ti/N 3.5 (see FIG. 7).

The impurities in the present invention include phosphorus, sulphur and so on, and the content of the imprities is less than 0.035%, and do not includephosphorous and sulphur added as alloying elements.

Next, in the present invention, the claims define the scope of application of suitable for a large heat-input welding more than 50,000J/cm. Such limitation of application is based on the fact that the present inventive steel shows remarkably good toughness of weld bonds when a large heat-input welding is applied, as com-- pared with a conventional steel.

In the present inventive steel, the discovery of such new properties and the development of such a new application are important limitations. These limitations e 6.0kg-m). This characteristic is assured when the heat-input is more than 50,000.l/cm and the heat-input is still larger.

Further, in case the heating condition before the rolling is that the frequency of heating above 1,100 C is not more than one time, the value of ,E of the reproduced heatcycle test show more than 7.5kg-m. Also even when the cooling condition of steel ingot is not limited and the heating condition thereafter is not limited, more than Sig-m of E, value of the reproduced heat-cycle is assured.

It is particularly note-worthy that when the welding heat-input is more than 70,000J/cm, ,E,, (absorbed energy at C) of reproduced heat-cycle charpy equivalent to a single layer bond shows a particularly excellent vlaue. And the structure of this bond is not martensite or buinitic, but a chromium, structure containing proeutectoid ferrite. When the heat-input is less than .l/cm, the charpy absorbed energy of the weld bond is slightly better than that of a conventional steel, but the effect is not so remarkable.

The weld may be deviced into the deposited metal, the bond (rough grained heat-affected zone) the fine grained heat-affected zone and the material to be welded, and as is well known the toughness is worst in the bond. For meaurement of the toughness in the bond, there are two methods; one method is to prepare a 2V notch charpy test piece from the actual welded joint, to notch the bond and to subject it an impact test; the other method is to give a test piece, a heat-cycle of maximum heating temperature of 1,350 l,450 C which is equivalent to the heat history given to the weld bond by means of a reproduction heat-cycle tester, to take a 2V notch charpy tet piece therefrom and to subject it to an impact test. Comparing the test results of these two methods, the result of the first method shows in general higher value that of the second method as is well known. The reason is that when the test piece is taken from the actual welded joint, a notch is given to portions near the fusion line, to the embrittled bond or to the fine grained heat-affected zone having high toughness, so that the toughness value of the weld bond measured by the first method shows often considerable variations and thus a relatively high average value is ob tained. While in the second method a heat-cycle similar to the heat history of the weld bond can be given correctly to the test pieceover a considerable range, and thus a notch is given completely in the weld bond and no effect of other portions is given thereto, so that the test value of the charpy test shows a low average value.

For example, when various conventionally known high tensile steels are tested for their bond toughness by the first method, ii {absorbed energy at 0 C by 2y charpy test) is 2 6 kg-m for a single layer welding of 50,000J/cm heat-input, but according to the second method, E, falls in the range of l 3 lkg-m without exception. Therefore, in the present invention, it is desirable to estimate the toughness strictly by the reproduced heat-cycle tester which give relatively low bond toughness value. And only by such a method for estimating the bond toughness the characteristics and excellent results of the present invention are clarified effectively.

The reproducted heat-cycle charpy test is briefly explained hereunder.

Heat-cycle same as that given to the actual weld bond to a square bar test piece from which a standard 2mmV notch charpy test piece as define by 11S can be prepared by means of a high frequency induction heating or a direct current passage heating. The test piece is heated rapidly to 1,350 1,400 C from room temperature in about 4 30 seconds depending on the capacity of the heating apparatus and the size of the test piece, and without being held at this temperature, is cooled along the cooling curve of the actual weld bond at various cooling rates corresponding to the welding heat-input. A 2mV notch charpy test piece is prepared from the heat-cycle given test piece and charpy testing is conducted at various testing temperature to determine the toughness of the bond.

The values of 2V charpy testings given in the present invention mean an average value of repeated tests of more than three test pieces.

The above explanations are given in cae of a single layer welding, and in case of a plural layer large heatinput welding, a part of the weld bond is tempered or normalized by the heat of the following weld beads, and toughness is considerably improved in case of a single layer welding. Therefore, in the present invention the toughness of weld bond is estimated by the toughness values when a reproduced heat-cycle test equivalent to the heat history of a single layer weld bond. This method is the most strict one, and when the minimum value of the bond toughness is assured by this method, better toughness values are guaranteed in all other cases. And in the conventional steels, the absorbed energy in a 2V charpy test is l 3kg-rn (,,E l 3kg-m) at 0 C at more than 50,000J/cm of heat-input, and the ductility-fracture ratio is less than 10% according to the method for estimating weld bond as disclosed in the present invention, whereas in case of the present inventive steel in which the solidification and cooling condition of the cast billet are limited, more than 6.0 kg-m of absorbed energy (vEo .0k g rn) at 0 C and more than 50% of ductility fracture ratio can be assured. Also in the present invention, when solidification and cooling conditions of the steel ingot and heating conditions thereafter are limited, ,,E Skg-m is assured. For the above reasons, when the toughness of weld bond is estimated by the charpy values obtained by a reproduced heat cycle test equivalent to the heat history of a single layer welding bond," the results of the present invention are most conspicuous.

In the present invention, 0.0001 0.006% of boron and/or 0.02 0.20% of vanadium are added to the basic chemical composition as defined hereinbefore. By the addition of 0.0005 0.006% of boron and/or 0.02 0.20% of vanadium the ,E value of weld bonds can be assured to be more than 6.0kg-m and the softening tendency of the welded portion can be avoided. These effects are small when the boron content is less than 0.0005 and the vanadium content is less than 0.02%, and deterioration of the toughness of the weld bond is caused when the boron content is more than 0.006% and the vanadium content is more than 0.20%. And thus the boron content is limited to 0.001 0.006% and the vanadium content is limited to 0.02

Further in the present invention, less than 5% of nickel, less than 2.0% of copper, less than 0.35% of chromium and less than 0.35% of molybdenum may be added singly or in combination. Ni, Cu, Cr and Mo may be added within a range in which the toughness of the weld bond can realize results as described in the present invention. Ni and Cu improve the toughness of both the weld bond and the material and also increase their strength. When the chromium and molybdenum contentsare less than 0.35% each the strength of both the welding material and the welded portion can be improved without lowering the toughness of the weld bond. I

The effects of the present invention will be explained under.

When a conventional high tensile strength steel is welded with a large heat-input, the toughness of weld bonds remarkably deteriorates, and as understood in Examples set forth hereinafter. When a reproduced heat cycle test equivalent to the heat history of a single layer weld bond with a heat-input of about 10,000 J/cm, for example, is conducted, the absorbed energy of the reproduced portion by the charpy test is l 3kg-m at C and the ductility fracture ratio is 0 9%. Whereas in the present invention in which solidification and cooling rate of the steel ingot or cast billet is limited to more than 5 C/min., an average absorbed energy of 6.0kg-m and a ductility fracture ratio of more than 50% can be assured by the same testing method.

The aboveproper'ties of the weld bond produced by a large heat-input in the present invention assure an absorbed energy of 5.0kg-m at -30 C according to other standards. In this case the proportion to the total steel of titanium present in TiN phase small than 0.051;. is more than 0.004% in the welding material before welding. Also when the solidification and cooling rate of the cast billet is more than 5 C/min. and at the same time the heating condition before rolling is that the frequency of heating above 1,100 C is not more than one time, the E, value of the reproduced heat-cycle charpy test is more than 7.5kg-rn (,,E,, 7.5kg-m), when the proportion of titanium in TiN smaller than 0.0511. is more than 0.006% in the welding material before welding. An when the frequency of heating at a temperature between l,050 C l,300 C is not more than one time, the E, value of the reproduced heat-cycle charpy test is more than 5.5kg-m (,,E 2 5.5kg-rn) irrespective to the cooling rate of the core portion of steel ingot or cast billet. Particularly in case the cooling rate of the core portion of steel ingot or cast billet is less than 5 C/min., the ,E value of the reproduction heat-cycle charpy test is more than 6.0kg-m (,E, 6.0kg-m) and the proportion of titanium in TiN smaller than 0.05p. is more than 0.004% so far as the frequency of heating at a temperature range of l,050 C l,250 C is not more than one time.

Further in a conventional high tensile strength steel,

the heat-input is limited due to the lowering of the weld bond toughness, and usually the upper limit is 45,000 to 50',000J/cm.

Whereas in the present invention the toughness of weld bonds is remarkably excellent at a heat-input more than 50,000J/cm, and although the lower limit of the heat-input condition is 50,000J/cm, the upper limit welding. This means the present inventive steel is completely different from the conventional steel in the mechanism of improvement of the weld bond toughness. 7 i

This mechanism is not'yet clarified but in a conventional steel the more the low-carbon martensite or low bainitic structure is, the better the toughness is, whereas in the present inventive steel, the more the fine ferrite is in the weld bond, the better the weld bond toughness is, and excellent toughness is obtained when the ferrite content is preferably more than 40% by area. Based on the conventional knowledges, in case of a steel composition in which low-carbon martensite or low bainite is formed even at a large heat-input welding, the weld bond toughness is good, but in this case a large amount of alloying elements must be added. When large amount of alloying elements are added, the socalled carbon equivalent is remarkably increased and weldability is considerably deteriorated, Whereas in the present inventive steel as the weld bond toughness is very excellent even without the addition of a large amount of alloying elements,'the carbon equivalent is remarkably low and the weldability is excellent. This is one of the important results of the present invention.

In FIG. 6 shows how the absorbed energy of 2mmV notch charpy test at 0 C is changed by the content of titanium in TiN smaller than 0.05;/. in the welding material before welding when a reproduced heat cycle equivalent to a welding heat-input of l00,000J/cm is given (maximum heating temperature: 1,400 C). It is clearly understood from the figure that the content of titanium in TiN smaller than 0.05; must be more than 0.004% in respect to the whole steel in order'to assure more than 6kg-m of absorbed energy of reproduced heat-cycle charpy of l00,000J/cm. The figure is shown as an average value of many tests.

The features and results of the present invention will be better understood from the following Examples.

EXAMPLE 1 in this example, the effect of the condition of solidification and cooling of the ingot or billet and the condition of heating thereafter ultimately on the Charpy value of the steel in the reproduced heat cycle test corresponding to a single layer welding with a heat input of 100,0001/cm is illustrated.

A steel within the composition range of this invention and a conventional steel as shown in Table 1 were used.

Under the solidification and cooling condition of the billet and the heating condition thereafter for the hot rolling as shown in Table 2, steel sheets with a thickness of 25mm were prepared. After hardening from 950 C and tempering at 630 C, the samples were treated with I a reproduced heat cycle corresponding to a single layer welding with a heat input of 100,000 .l/cm, and Charpy extends to electro-gas welding and even electroslag test was made with these samples.

TABLE l..-STEEL COMPOSITIONS (PERCENT) Ntotal C Si M0 P V Ti B Alsol Ninsol Inventive SteekA 0.12 0.10 1.45 0.020 0, Q0035 0,01% g-gggi Conventional t l: B 0.16 0.23 1.22 0.015 0, M27 8 TABLE 2.-RESULTS OF MATERIAL TESTING Numerical proportion of Ti compounds less Reproduced Average cooling than 10,000 A. heat cycle T1% 111 TiN less rate in billet in cast billet, Starting material charpy value than 0.05; in core (pouring percent vEo correspondmaterial before 0.) Lowest temp. of rapid cooling of cast (electron rnicro- QB vTrs ing to 1000001 Heating condition welding (weight C./min.) billet C.) scope X10,000) (kg/cm?) 0.) cc, kgJm. before rolling percent) a e m Tmomemwm m 22;: as tee 815212. 2 8:85? 65.3 -42 6.2 1,250: o. 2 times 0.00s To l,050 C. (slow cooling thereafter)- 6O I2 iggs iggfi: g: 65.1 -49 13.1 1,150 c. 1 time... 0.010 To 1,175 C. (slow cooling thereafter)" 45 If H23: fag? 8g;

To room temperature 25 l? 2:2 $33. 8: f 2E3: 81% B: 65. 1 52 2.3 1 250 C. 2 times... temperamre 0 i 66.3 -48 2.1 1Z203 o. 1 time.... 0 To 1,050 0. (slow cooling thereafter)" 0 :3 i'igg: 23 g 5.2 To 1,175 0. (slow cooling thereafter)" 0 gg'g g3 Egg 3 1- To room temperature 0 $12 2?, Z; 3 5 23. 8; f 1 ,331; 8

As it is obvious from the results in Table 2, the steel within the composition range defined in this invention shows generally a better Charpy value than the conventional steel for the bond part in the welding with 100,000 .l/cm. Particularly, when the billet is quenched below l,l00 C with a mean cooling velocity of slower than 5 C/min. at the central part of the billet in the range from the pouring (higher than l,550 C) to l,l00 C, and heated only once higher than 1,100 C in the re-heating, the Charpy value of bond part in the reproduced heat cycle Charpy test is very high. Such an effect could not be obtained at all in the conventional steel.

To evaluate the toughness of the welding bond part, besides said reproduced heat cycle test, submerged welding was carried out practically with a heat input of 100,000 J/cm, and Charpy test was made with a test piece taken from the actual bond. Except that the absolute values of absorption energy were generally 4 6kg-m higher than the values in the reproduced heat cycle test, the tendency was quite similar to the abovementioned result.

According to the electron microscopic observation of numerous extraction replica samples, it was ascertained that, when the Charpy value of bond part was high, fine deposits, smaller than about 1,000A, existed more than 50% in the numerical ratio among the total number of Ti-compound, at intervals of about 2 5p. in the steel master sheet. Some examples of the electron microscopic photograph are shown in FIG. 4. In the photograph, square or rectangular deposits are Ticompound and it is clear that numerous fine particles of Ti-compound, smaller than 1,000A, are found only in the inventive steel A. On the other hand, the structure of bond part in the actual welding with a heat input of 100,000 J/cm is shown in FIG. 3. It is obvious that the steel in the composition range defined in this invention has a very fine structure as compared with the con- EXAMPLE 2 In this example, the master-sheet, hardened and tempered as a heat treatment thereof, is illustrated. Generally hardening was carried out from a temperature a s fi i lz lliClaami riewsie iee 9 at a temperature range of 500 690 C.

In Table 3, steels l 11 are the examples of this invention, and steels 31 33 are conventional refined high tension steels. Steels 12 30 are examples without the composition range of this invention, shown as for comparison. The object of comparison is to show how steels without the composition range of this invention have defects as a high tension steel for the use of automatic welding, and to clarify, as a consequence, the reason for defining the constituents in this invention.

In these examples, the cooling condition of the billet was: mean cooling velocity from the pouring to l,lO0 C was about 10.7 C/min. at the central part of the billet, and below l,l00 C the billet was cooled spontaneously in air to the room temperature. Then the billet was soaked once at l,200 C, and was rolled to obtain steel sheet by the hot rolling. After hardening from 950 C and tempering with at 620 650 C, properties of the master-sheet were tested, and the hand welding test, the reproduced heat cycle Charpy test with a heat input of 105,000 J/cm as well as the welding joint test (tensile test and Charpy test) were carried out. The data are as shown in Table 4.

$ 1 ...99 9 ?91 1i.if

81 M10 P 'li A1801 Inventive H OkOCDNJGWFWN Steels Comparative Steels No. N total B V Nb Ni Cu Cr Mo Inventive Steels Comparative Steels P Ti A1 801 N sol Ni; nsol Comparative $120618 0 onverition'al Ste e1 No. N total B V Nb Ni Cu Cr Mo Compa'r'ative Steels Gonventional Steels Conventional Steel Properties of starting material Properties of automatic Manual welding bonds 105000.1/66 We'ldability at 18000 J/cc (single layer welding) strength strength welding.

2 0.2% 2 o Reproduced heat Tensile kg/cm kg/cm cycle charpy testing value Ceq Hmax VEO Ductility xTensile Elonfracture strenggakg-m ratio 96 the tion Inventive Steels Comparative Steels Properties of automatic Manual weldability Propertles welding bonds 1o5oooJ/66 at l8000J/cc i No. starting material (single layer welding) welding Reproduced Iensile Z Ylela VTI'S heat cycle testing rang- Streng' o Charoy value th 2 til-60.2% C r Csq El kg/cm kg/cm E Ductili- Tensile Elonty fracstrenggature th 2 tion .k;;-m ratio 96 kg/cm 96 Comparative Steels Conventional Steels Conventional Steel To compare the inventive steels l 11 with conven-' tional steels 31 33, it is obvious that the inventive steels are unexpectedly excellent in the toughness of welding bond to the single layer automatic welding with 5 reproduced heat cycle bond of the inventive steels exceeded 5.0kg-m. For these inventive steels, .single layer submerged welding with a heat input of 105,000 J/cm was carried out practically, and an absorption energy of 14 26kg-m, as E was obtained for the Charpy test pieces obtained from their joints.

Steel 12, whose C content is without the range of this I invention, is lacking the strength of the master-sheet as well as the joint part in the automatic welding with 105,000 J/cm. While the content of any component of 

2. A high tensile strength steel for use in a welding process wherein the heat-input is greater than 50,000 J/cm, and which contains fine titanium compounds having a particle size of less than 1,000 A in an amount greater than 50 percent of the total amount of titanium compounds present and which comprises 0.03 to 0.23% carbon, 0.02 to 0.8% silicon, 0.5 to 2.0% manganese, 0.004 to 0,07% of titanium, 0.0005 to 0.10% of soluble aluminum, and 0.003 to 0.011% of total nitrogen, wherein the ratio Ti/N is < or = 3.5, with the balance being iron and impurities.
 3. A high tensile strength steel for use in a welding process wherein the heat input is greater than 50,000 J/cm, and which contains fine titanium compounds having a particle size of less than 1,000 A in an amount greater than 50 percent of the total amount of titanium compounds present and which comprises 0.03 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0% of manganese, 0.004 to 0.07% of titanium, 0.0005 to 0.10% of soluble aluminum, and 0.003 to 0.011% of total nitrogen, wherein the ratio Ti/N is < or = 3.5, with addition of at least one of 0.0005 to 0.006% of boron and 0.02 to 0.2% of vanadium with the balance being iron and impurities.
 4. A high tensile strength steel for use in a welding process wherein the heat input is greater than 50,000 J/cm, and which contains fine titanium compounds having a particle size of less than 1,000 A in an amount greater than 50 percent of the total amount of titanium compounds present and which comprises 0.03 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0% of manganese, 0.004 to 0.07% of titanium, 0.0005 to 0.10% soluble aluminum, 0.003 to 0.011% of total nitrogen, wherein the ratio Ti/N is < or = 3.5, with addition of at least one of 0.0005 to 0.006% of boron and 0.02 to 0.20% of vanadium and still further addition of at least one of less than 5% of nickel, less than 2.0% of copper, less than 0.35% of chromium, less than 0.35% of molybdenum, with the balance being iron and impurities.
 5. A process for producing a high tensile strength steel for use in a welding process wherein the heat input is greater than 50, 000 J/cm, which comprises cooling to below 1,100* C, a steel ingot or cast billet which contains 0.03 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0% of manganese, 0.004 to 0.07% of titanium, 0.0005 to 0.10% of soluble aluminum and 0.003 to 0.011% of total nitrogen, wherein the ratio of Ti/N is < or = 3.5 and at least one element selected from the group consisting of up to 0.006% boron, up to 0.2% vanadium, up to 5% nickel, up to 2% copper, up to 0.35% chromium, and up to 0.35% molybdenum, under a cooling condition such that the average cooling rate, from the temperature of the molten steel at pouring to 1,100* C is more than 5* C/min., and then hot working the steel ingot or cast billet.
 6. A process for producing a high tensIle strength steel for use in a welding process wherein the heat input is greater than 50, 000 J/cm, comprising using a steel material containing 0.003 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0% of manganese, 0.004 to 0.07% of titanium, 0.0005 to 0.10% of soluble aluminum and 0.003 to 0.011% of total nitrogen, wherein the ratio Ti/N is < or = 3.5 and heating the steel material between 1, 050* and 1,300* C only one time after the casting.
 7. The process according to claim 6, in which the steel material further contains at least one element selected from the group consisting of up to 0.006% boron, up to 0.2% of vanadium, up to 5% nickel up to 2% copper, up to 0.35% chromium, and up to 0.35% molybdenum.
 8. A process for producing a high tensile strength steel for use in a welding process wherein the heat input is greater than 50, 000 J/cm, comprising casting a steel material containing 0.03 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0 % of manganese, 0.004 to 0.7% of titanium, 0.0005 to 0.10% of soluble aluminum and 0.003 to 0.011% of total nitrogen, wherein the ratio Ti/N is < or = 3.5, into the form of a steel ingot or cast billet, cooling said form to below 1,100* C such that the average cooling rate at the core portion, from the initial temperature of the molten steel at pouring to 1,100* C, is more than 5* C/min., and heating the form above 1,100* C only one time thereafter.
 9. The process according to claim 8 in which the steel material further contains at least one element selected from the group consisting of up to 0.006% boron, up to 0.2% vanadium, up to 5% nickel, up to 2% copper, up to 0.35% chromium, and up to 0.35% molybdenum.
 10. A process for producing a high tensile strength steel for use in a welding process wherein the heat input is greater than 50,000 J/cm, comprising casting a steel material containing 0.03 to 0.23% of carbon, 0.02 to 0.8% of silicon, 0.5 to 2.0% of manganese, 0.004 t0 0.007% of titanium, 0.0005 to 0.10% of soluble aluminum and 0.003 to 0.011% of total nitrogen wherein the ratio Ti/N < or = 3.5, into the form of a steel ingot or cast billet, cooling the form such that the average cooling rate at the core portion, from the temperature of the molten steel at pouring is less than 5* C/min., wherein the ingot or billet is not heated above 1,250* C and is heated to a temperature range from 1,050* to 1,250* C only one time after said cooling.
 11. The process according to claim 10 in which the steel material further contains at least one element selected from the group consisting of up to 0.006% boron, up to 0.2% vanadium, up to 5% nickel, up to 2% copper, up to 0.35% chromium, and up to 0.35% molybdenum. 